The overall objective of the proposed research is to develop two methods of testing the viability of preimplantation mammalian embryos. The first is to measure the rate of uptake of metabolic substrates, either glucose or pyruvate, by single preimplantation embryos incubated in a microdroplet of medium. The substrates will be estimated by a very sensitive ultramicrochemical method which allows measurements in the picogram per hour range. The possibility of developing a functional test based on stimulating the uptake of glucose by exogeneous hormones or sera will also be examined. The second approach is to monitor the intracellular pH of individual cells of preimplantation embryos. The pH will be monitored by means of pH sensitive fluorogenic dyes detected by microfluorometry. A functional test will be based on whether the cells can regulate their intracellular pH by exposing them to either alkaline or acid loads. If criteria for selective embryos for transfer can be formulated, their effectiveness will be tested by embryo transfer experiments. The mouse and the rabbit will be used as animal models.